The present invention relates to an error correction technique in a digital data transmission system, and in particular, to an encoder having an error correcting operation, a decoder having error correcting operation, and a data transmission apparatus using the encoder and decoder operating in a digital multilevel modulation system.
For mobile and terrestrial digital radio communication, there have been heretofore put in use data transmission systems using digital multilevel modulation systems such as a 16 or 64 quadrature amplitude modulation (QAM).
In a prior art digital multilevel amplitude modulation system, transmission data to be transmitted is converted into multilevel data using, for example, an encoder as shown in FIG. 1.
In the example of conversion to multilevel data by the encoder as shown in FIG. 1, to decide coordinates of signal points multilevel-converted in accordance with transmission data on a two-dimensional plane of signal points including an I-axis for real numbers and a Q-axis for imaginary numbers, transmission data in a serial form is inputted to a seal/parallel converter 1A and is converted into data in a parallel form.
For example, in the 16 QAM system, the signal point plane is represented on a constellation plane as shown in FIG. 2. The plane includes 16 signal points each of which is defined by values of a 4-bit transmission data signal. In this case, therefore, the serial/parallel converter 1A produces the signals having 4 bits.
Consequently, the serial/parallel converter 1A includes four registers 7 as shown in FIG. 1. Output signals of respective registers 7 are sequentially transferred through the registers every predetermined clock signal period and are fed to respective latches 51 every clock signal period for 4-bits. 4-bit parallel data is resultantly inputted to a signal point generator 3A. The generator 3A accordingly decides signal points on the plane shown in FIG. 2 and outputs amplitude values respectively on the I-axis component, namely, in-phase amplitude component, and the Q-axis component, namely, quadrature amplitude component, to roll-off filters 4 and 5 (ROF: hereinafter referred to as filter), respectively. Signals outputted from the filters 4 and 5 are fed to transmission process circuit of a succeeding stage.
In the signal point generator 3A, of the inputted 4-bit data signal, respective bits of 2 bits thereof are made in-phase component I1 and I2, and respective bits of the remaining 2 bits are made quadrature components Q1 and Q2, and signal points on the constellation plane are decided in accordance with combination of the values of the respective components I1, I2, Q1 and Q2 as shown in FIG. 2.
Assuming, for example, that the values of the components I1, I2, Q1 and Q2 are 0, 0, 1, and 0. Then, the point of the coordinate (+1, +3) on the constellation plane is the signal point corresponding to the combination of the component values 0, 0, 1, and 0.
In this way, a signal indicating I-axis coordinate +1, which indicates that the in-phase amplitude component level is +1, is fed to the roll-off filter 4 and a signal indicating Q-axis coordinate +3, which indicates that the quadrature amplitude component level is +3, is fed to the roll-off filter 5.
The technique of the quadrature amplitude modulation of the roll-off filters 4 and 5 are not directly related to the present invention and hence description thereof will be omitted.
A 16 QAM system has been employed as an example of the multilevel modulation system. However, in other multilevel modulation systems, the configuration above is basically applicable. Only the number of signal points and the number of bits to define the signal point differ between the systems.
The multilevel transmission data will be thereafter transmitted from the transmitting apparatus to a receiving apparatus. A general technique regarding the multilevel signal generation and a general technique for subsequent modulation and transmission have been described in detail in pages 82 to 89 of xe2x80x9cTHE THEORY AND PRACTICE OF MODEM DESIGNxe2x80x9d written by John A. C. Bingham and published from A WILEY-INTERSCIENCE PUBLICATION in 1988. Therefore, the techniques will not be described in detail. The disclosure of the document is hereby incorporated by reference herein.
In the data transmission system as mentioned above, in a case in which factors to deteriorate transmission data to be transmitted in the transmission path are increased, when the transmission data is received by the receiving apparatus, the received data is erroneous data which is considerably different in the data value of the original transmission data generated at the transmitting apparatus. The data receiving apparatus cannot easily reproduce the original transmission data and a problem of the reproduced data being erroneous occurs.
In a known method, in reproducing transmission data received, it is reproduced by convolutional encoders and Viterbi decoders by minimizing occurrence of data errors to a possible extent. However, in a transmitting unit corresponding to the data reproduction method, it is necessary that serial transmission data is converted into parallel data n bits by n bits and thereafter one convolutional encoder is used for each bit of the parallel data to generate an error correction code. For example, in a 16 QAM system (n=4), one convolution coder is employed for each bit of parallel data of 2-bit length. The encoder encodes data of one bit of the 2 bits to generate an in-phase two-bit component (I) signal and encodes data of the other bit of the 2 bits to generate a quadrature two-bit component (Q) signal. A rate between the number of bits of data before convolution and that of data after convolution is called xe2x80x9ccoding ratexe2x80x9d. In the example, since two convolutional encoders are used to encode a 2-bit input signal to generate a 4-bit signal in the overall system, the coding rate is 2 bits/4 bits, that is, xc2xd. The coding rate is associated with a data transmission rate. In the case where the coding rate is as small as 50% as in the example, the data transmission rate is correspondingly lowered, thereby reducing the transmission efficiency.
Since the configuration of FIG. 1 does not include an error correcting function, the transmission rate of the transmission path is equal to the data rate of the transmission data, namely, the coding rate is 1, transmission efficiency is not lowered. In the case where however an error correcting function is provided, transmission efficiency is reduced because of the coding rate.
In the case where the error correcting function is not disposed making the coding rate 1, the possibility that the data transmission efficiency falls is few. However, data is likely to be influenced by, for example, noise on the transmission path. This leads to an undesirable result that bit errors occur in reception data received.
On the other hand, in the case where the error correcting function is added, possibility of being influenced by noise on the transmission path can be minimized and occurrence of bit errors in reception data can be lowered. However, the data transmission rate is reduced because of the coding rate.
Assuming, for example, that the transmission system has a data transmission rate of 54 megabits per second (Mbit/s) on transmission path, the data transmission rate is lowered to 27 Mbit/s when the coding rate is xc2xd, when considered in terms of data amount before convolutional encoding.
For error correction, since the transmission side requires convolutional encoders and the reception side requires Viterbi decoders, the encoder and the decoder must be more efficiently operated to improve the coding rate.
It is therefore an object of the present invention to provide an encoder having an error correcting function, a decoder having an error correcting function, and a transmission apparatus using the encoder and decoder in which the coding rate is increased by an error correcting function and hence data transmission efficiency is improved.
To achieve the object, there is provided in accordance with one aspect of the present invention an encoder having an error correcting operation for use in a transmitting unit which sends transmission data in a 2n QAM system (n is an integer equal to or more than four). The encoder includes a converter section to convert the transmission data into an (nxe2x88x921)-bit parallel data and a convolutional encoder to receive one bit of the (nxe2x88x921)-bit data. The encoder produces 2-bit data. The 2-bit data from the convolutional encoder is separated into a 1-bit in-phase component and a 1-bit quadrature component. Using the remaining bits of the (nxe2x88x921)-bit data and the 2-bit data from the convolutional encoder, multilevel signals are generated according to the 2n QAM system.
In the case where n is an even number equal to or more than four, the multilevel signals of the 2n QAM system may be configured as below.
In a signal point plane of in-phase amplitude components (I) and quadrature amplitude components (Q), an identical bit layout is assigned to in-phase components at signal points having an identical in-phase amplitude component and an identical bit layout is assigned to quadrature components at signal points having an identical quadrature amplitude component.
Mutually different bit values, i.e., 0 and 1, are alternately assigned to in-phase component bits corresponding to an in-phase component output from the convolutional encoder for signal points adjacent in the I-axis direction to each other.
Mutually different bit values, i.e., 0 and 1, are alternately assigned to quadrature component bits corresponding to a quadrature component output from the convolutional encoder for signal points adjacent in the Q-axis direction to each other.
In the case where n is an odd number equal to or more than five, the multilevel signals of the 2n QAM system may be configured as below.
The signal point plane of in-phase amplitude components (I) and quadrature amplitude components (Q) is subdivided into a first area and a plurality of second areas.
In the first area, an identical bit layout of in-phase component is assigned to in-phase components at signal points having an identical in-phase amplitude component and an identical bit layout of quadrature component is assigned to quadrature components at signal points having an identical quadrature amplitude component.
Bit layouts other than those assigned to the first area are assigned to signal points in the second areas.
Throughout the first and second areas, mutually different bit values, i.e., 0 and 1, are alternately assigned to in-phase component bits corresponding to the in-phase component output from the convolutional encoder for signal points adjacent in the I-axis direction to each other. Mutually different bit values, i.e., 0 and 1, are alternately assigned to quadrature component bits corresponding to the quadrature component output from the convolutional encoders for signal points adjacent in the Q-axis direction to each other. In the case where the direction in which the adjacent signal points crosses a boundary between the first and second areas, the order of xe2x80x9c0xe2x80x9d and xe2x80x9c1xe2x80x9d is kept unchanged. In the case where the direction in which the adjuacent signal points extend is parallel with the direction of a boundry between the first area and the second area, the order of xe2x80x9c0xe2x80x9d and xe2x80x9c1xe2x80x9d is adjusted to be equal to that in the adjoining first area. The bit layout must be unique to each signal point.
Favorably, in the case where bit layouts not assigned to the first area are assigned in the second area, an identical bit layout of in-phase component is assigned to in-phase components at signal points having an identical in-phase amplitude component as available as possible from the unassigned bit layouts and/or an identical bit layout of quadrature component is assigned to quadrature components at signal points having an identical quadrature amplitude component as available as possible from the unassigned bit layouts.
In accordance with another aspect of the present invention, there is provided a decoder having an error correcting operation for use with 2n QAM signals (n is an integer equal to or more than four). In the decoder, a signal point plane of an in-phase amplitude component (I)xe2x80x94quadrature amplitude component (Q) coordinate system is divided in directions respectively of in-phase and/or quadrature amplitude components into a plurality of areas. The decoder includes an area deciding section for receiving an in-phase amplitude component (I) signal and a quadrature amplitude component (Q) signal of the QAM signals, the section deciding to which ones of the plurality of the divided areas each of the in-phase amplitude component (I) and quadrature amplitude component (Q) signals belongs, respectively; a metric assigning section for producing, according to a result of decision by said area deciding section, metric for each of the in-phase amplitude component (I) and quadrature amplitude component (Q) signals, a Viterbi decoder for receiving the metric for the in-phase amplitude component (I) and quadrature amplitude component (Q) signals, a convolutional encoder for receiving a data signal outputted from said Viterbi decoder, and a parallel/serial converter for receiving an output from said Viterbi decoder and outputs from the convolutional encoder.
In accordance with another aspect of the present invention, there is provided a data transmitting apparatus having the error correcting function. The apparatus includes a transmitter section including the encoder with error correction and a receiver section including the decoder with error correction.